New Ordovician bivalves from the Indochina Palaeoplate in Dali, western Yunnan, Southwest China and their palaeogeographic significance

A centre of radiation for Ordovician bivalves is identified based on an abundant and diverse fauna from the Hsiangyang Formation (Darriwilian, Middle Ordovician) of Dali, western Yunnan, Southwest China. It consists of 22 species of 18 genera including 1) one new genus and four new species: Rhomboconcha tresdentes n. gen. n. sp., Fasciculodonta curvata n. sp., Glyptarca symmetrica n. sp., and Paracyclas initium n. sp.; 2) three taxa known from elsewhere but initially found in Dali: Praeleda sp., Glyptarca sp., and Redonia deshayesi; 3) other taxa previously reported by Fang and Cope (2004). Numerical analysis of the distribution of 11 Middle Ordovician bivalve faunas from different regions shows two distinct faunal groups, the HPL group, representing the bivalves that lived in higher palaeolatitudes, and the LPL group (including Australia and South China) in lower palaeolatitudes. The bivalve fauna from Dali correlates with both groups, indicating that the Indochina Palaeoplate was located between these two groups, and in middle–high palaeolatitudes. Thus, the Indochina Palaeoplate was probably an isolated centre for bivalve radiation during the Middle Ordovician; those genera confined to Dali did not then spread to other palaeoplates.